Last time I had a problem with a lens, Olympus suggested this test with a broadsheet newspaper page for a target.

I also found a row of distant houses is also a good target for testing for decentering when focused at infinity.

tt321 wrote:

...

You can do the upside down test.

At close focus this is somewhat easy - you can use a computer screen with a lot of text, at longer focusing distances you need to find appropriate large targets.

With a computer screen, set the camera close enough so that it's entire view is filled with text. Now try to set the lens as perpendicular to the screen as possible so that the text does not converge/diverge from left to right or from top to bottom - parallel text should run parallel. Focus at the centre spot. Shoot a picture. Note down five control characters (letters, numerals, punctuation, etc.), one at each corner of the frame and the last one at the dead centre that was used to focus the shot. Depending on the size of the text you may need to use groups of characters for these five reference points. Turn the camera upside down. Let's assume these characters are A, B, C, D for the corners from upper left in a clock-wise direction, and O for the centre. Maintain the same (as same as you can) relative position of camera vs. screen with being upside down being the only difference. Shoot with the exact same framing, i.e. the same characters in the four corners, and position O in the centre to focus at.

Now look at these two pictures at 100% pixel detail. First check the centre focus character to make sure that both pictures are focused correctly. Then examine the corners. If the lens is not centred properly, you will see inconsistent corner sharpness at different corners, e.g. A sharp and C unsharp in one shot and C sharp and A unsharp in the other.

Or you can make sure that you position the sensor parallel to the target with a small mirror fixed to the centre of the target - better with a large poster rather than a computer screen. Right-on aiming should show the camera (ideally the centre of the lens at the centre) in the mirror when you are aiming. If a shot is taken and one side or one corner is not sharp whilst others are, there is a problem with the lens.

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